1. Field of the Invention
The present invention relates generally to axial flow fans, and more particularly to improvements in an engine cooling fan, an air ventilating fan, an air conditioning fan, and other axial flow fans.
2. Description of the Prior Art
Recently, there has arisen a demand for increasing the airflow or the flow rate of a fluid being impelled by a fan, such as, for example, for use in an engine cooling system of a motor vehicle, due to increases in heat generation and heat capacity as a result of the provision of additional equipment, such as, for example, exhaust-gas processing equipment, a cooler, power steering, and the like, and due to an increase in engine power required. At the same time, a demand for reducing the noise attributed to the fan has also been made.
Still further, a cooling fan used in air conditioning equipment is subjected to strict requirements due to an increase in the capability required per unit volume of the equipment, and consequently, a demand for reducing the weight and size of the equipment has also arisen.
A typical prior art axial flow fan will now be described herein by reference to a fan normally used in an engine cooling system of a motor vehicle in conjunction with the accompanying drawing FIGS. 1-3.
A cooling fan 4 provided on the side of an engine 3 in an engine compartment 2 of a motor vehicle 1, only part of which is shown, is coupled by means of a belt-pulley system 5 to the output shaft 6 of the engine 3.
From the front end of the motor vehicle toward the cooling fan 4, yet in front of the cooling fan 4 as viewed in the direction of travel of the motor vehicle, there is also provided a radiator grill 7, a condenser 8, and a radiator 9, in that order, respectively.
In general, the aforenoted cooling fan 4 is of the suction type and hence rotates in a direction so as to produce suction, whereby, due to the rotation of the cooling fan 4 in the direction of the arrow 10, there is produced air streams 11A and 11B on the blades of the fan, whereby there is also produced cooling air streams 12 passing through the aforenoted grill 7, condenser 8, and radiator 9.
Secured integrally to the radiator 9 is a shroud 13 having a slightly larger inner diameter than the locus of the maximum diameter of the fan blades 18 of fan 4, shroud 13 thereby covering the cooling fan 4. This is provided because all of the air streams which have been introduced under suction conditions by means of the fan 4 are desired to pass through radiator 9, with an accompanying increase in the capacity of the fan which blows the air. In this respect, there is provided a clearance S between the maximum diameter of the fan blades 18 and the inner periphery of the shroud 13. This clearance prevents contact between the fan 4 and the shroud 13 due to their relative vibrations, because the fan 4 is attached to the side of the engine 3, and hence vibrates together with the engine, while the shroud 13 is rigidly fixed on the side of the radiator 9. The clearance is generally set to be about 20 mm.
On the other hand, there is also provided clearances of about 20-30 mm between the fan 4 and a portion adjacent to the fan, such as, for example, between the fan 4 and the engine 3, radiator 9, or the like, because the fan may possibly contact these portions due to an axial movement of the fan which may be caused by a sudden stop or start of the motor vehicle, or due to a deformation of the fan, for which heat, stress, and the like, may be responsible.
When a motor vehicle is running, the vehicle receives ram air 14 from the front thereof, as shown in FIG. 3, so that the cooling action may be achieved due to the aforenoted ram air streams 14 as well as due to the cooling air streams 12 produced by the rotation of the fan 4.
The larger the clearance S between the fan 4 and the shroud 13, the better will be the efficiency of the ram air streams 14. However, if the clearance S is relatively large, and in case the ram air steams 14 are not prominent, as in the case of idling and low speed running of the motor vehicle, then there results a pressure difference between the suction side and the discharge side of the fan 4, with the result that there is provided a reverse flow of air from the discharge side of the fan, through the clearance S, to the suction side thereof, thus lowering the ventilating efficiency of the fan 4 to a great extent, as compared with the case where the clearance S is relatively small. This in turn lowers the blowing capability of the fan 4, and for these reasons, it is not recommended to provide a large clearance S.
The dimensions of the engine compartment have been minimized due to limitations from the viewpoint of design and style of the vehicle, as well as due to the requirements for compactness and decreased weight, and because of the installation of exhaust-gas processing equipment, accessories, and the like. For this reason, the efficiency of the cooling system having the aforenoted construction and function has been impaired, and yet, due to limitations either in the radial direction or in the axial direction, it has been difficult to increase the size of the cooling fan, despite a demand for an increase in the cooling capability thereof.
Generally, an increase in the airflow of an axial flow fan may be achieved by (i) an increase in the RPMs of the fan, (ii) improvements in the dimensions thereof, such as, for example, increases in the diameter of the fan, the blade angle, the number of blades, and the width of the blades, and (iii) a change in the configuration of the blades.
An increase in the rpm of the fan, however, necessarily leads to an increase in the horsepower required to drive the fan, as well as in an increase in the noise level produced, while improvements in the dimensions of the fan, and the blade configuration thereof results in an increase in the volume of equipment and in the level of noise. In addition, such increases cannot necessarily be attended with a proportional increase in airflow. On the other hand, one solution to this problem may be to reduce the clearance between the fan and the shroud, however, this suffers from various shortcomings, such as, for example, the danger of contact therebetween, as has been described hereinabove, and a decrease in the effect of the ram air streams.
Accordingly, it has long been desired to provide an axial flow fan which facilitates an increase in the airflow without accompanying changes in the dimensions and rpm of the fan and an increase in noise, or which may in fact reduce the noise level under a constant airflow, yet present a high level of efficiency in such an instance where there prevails a large blowing resistance. Such a desired axial flow fan, therefore, will be one in which the air streams created by the prior art fan will be utilized most effectively.
For a better understanding of the air streams created by the prior art axial flow fan, a more detailed description will now be given hereinbelow, by again referring to the engine cooling system.
A very large blowing resistance prevails on the suction side of the cooling fan 4, because of the decrease in the area of the opening of the radiator grill 7 which is required from a design consideration, the provisions of the condenser 8 due to the provision of a cooler, a decrease in the fin pitch for improving the radiating characteristics of the radiator 9, and an increase in the louver angle. A large blowing resistance also prevails on the discharge side of the fan 4, because of the engine block 3, exhaust gas processing equipment, a power steering pump, a cooler-compressor, and the like.
In case the fan is rotated in such a position wherein the large blowing resistance prevails on both the suction and discharge sides of the fan, air which is flowing in the axial direction on the suction side of the fan flows out in the inclined radial direction on the discharge side of the fan, as shown by the arrow 15 in FIG. 1, and this tendency becomes substantial as the blowing resistance increases on one or both of the suction and discharge sides of the fan.
This is attributed to the fact that, on the suction side of the fan where a large blowing resistance prevails, there is created a negative pressure or vacuum in a chamber 16 encompassed by fan 4, radiator 9 and shroud 13, while there prevails a slightly, positive pressure in a chamber 17 defined between the fan 4 and the engine 3 on the discharge side of the fan, due to the pressure build-up caused by the rotation of the fan and by the blowing resistance, whereby air streams will be slightly deflected toward the side of the lower pressure, and, as a result, will flow in the inclined radial direction as mentioned above.
On the other hand, air streams flowing along the surface of one of the blades 18, that is, air streams 11A flowing in the direction opposite to the direction 10 of the rotating blade and at the leading edge 18A thereof, also tend to be deflected in an inclined radial direction within the vicinity of the trailing edge 18B of the blade, thereby creating air streams 11B, as shown in FIG. 2, so that there occurs separation of the air streams from the blade surface, or in other words, turbulent air streams therealong. This incurs an increase in the aerodynamic loss of the fan and a lowering of its efficiency which in turn brings about a lowering of the generated airflow.